In fact, some HTTP server will send "Expires: -1" in HTTP response header.
You are right, HTTP spec is that invalid value, include "0".
I add a test to test_http_connection() that would send "Expires: -1" in response header, I get the error log:
``` 0024:err:wininet:HTTP_ParseDateAsAsctime unexpected weekday L"-1" 0024:err:wininet:HTTP_ParseDate unexpected date format L"-1" ```
Maybe we should just treat "Expires: -1" as an invalid date format and ignore it.